save_map.vim

nnoremap <leader>f :call someFunction()<CR>
let map = "\<leader>f"
let save_map = maparg(map)
" save_map == ':call someFunction()<CR>'
nnoremap <leader>f :call someOtherFunction()<CR>

" I'd like to execute the previous mapping of <leader>f

" doesn't work because '<CR>' is not escaped in save_map
exe "normal!" save_map

switch.vim

if !exists('g:switch_mapping')
  let g:switch_mapping = 'gs'
endif

let g:switch_mapping_fallback = 1

command! Switch call s:Switch()
function! s:Switch()
  let definitions = s:GetDefinitions()
  " returns either 0 or 1
  let retval = switch#Switch(definitions, {}) 
  if g:switch_mapping_fallback && !retval
    exe 'normal!' s:orig_mapping
  endif
endfunction

if g:switch_mapping != ''
  let s:orig_mapping = maparg(g:switch_mapping, 'n')
  if empty(s:orig_mapping)
    let s:orig_mapping = g:switch_mapping
  endif
  exe 'nnoremap' g:switch_mapping ':Switch<cr>'
endif